New Evidence for Life on Mars?

by Ivan Noble

Scientists have published what they claim is conclusive evidence that bacteria
once lived on Mars.

In 1996 the American space agency Nasa announced the discovery of remnants
of ancient bacteria in a meteorite from Mars, but critics at the time said the
evidence was too thin. Now a Nasa-backed team from the US, Spain and Germany
has published new work strengthening the case.

But some British experts are skeptical, saying the study falls short of absolute
proof.

The argument centres on tiny crystals found in a potato-sized meteorite discovered
in Antarctica in 1984. The international team says the crystals are the same
as those deposited by earthly microorganisms known as magnetotactic bacteria.
Since the meteorite is from Mars, it can only be assumed that the same bacteria
must have lived on Mars as well, they say.

Signature of life

The crystals in question are made of magnetite, a compound of iron and oxygen.
The team's aim was to show that they must have been deposited by living organisms,
rather than a simpler chemical process. Magnetite crystals deposited by bacteria
on Earth show six key features that distinguish them from crystals of non-biological
origin. This biosignature is what researchers have been looking for. Last year
another group of researchers announced that they had found some, but not all,
of the six key features.

Crystal chain clue

Now, in a paper published in the journal Proceedings of the National Academy
of Sciences (PNAS), Imre Friedmann, Jacek Wierzchos, Carmen Ascaso and Michael
Winklhofer say they have now found the missing sixth element. The final clue
was the arrangement of the crystals in the meteorite. Crystals left behind by
bacteria on Earth form characteristic chains.

Dr Imre Friedmann of Nasa's Ames Research Centre says they used a new method
of electron microscopy to show that the Martian crystals are in chains, too.

"These crystals match all the criteria for biologically-formed chains,
and as far as I'm concerned it's conclusive evidence that Martian bacteria were
in this meteorite. I cannot guess how my colleagues will react, but in my opinion
no reasonable person can doubt it any more. The evidence is so strong. When
you put all the elements together there can be no other explanation," Dr
Friedmann told the Press Association news agency.

Mission to Mars

Professor Colin Pillinger, lead scientist in the British-led Beagle 2
effort to land on Mars in 2003, describes the NASA-backed study as very interesting,
but says it falls short of absolute proof.

"The bigger the claim, the more perfect the evidence has to be. This doesn't
actually prove that the evidence in the meteorite is for life on Mars,"
he told BBC News Online. "We cannot say absolutely hand on heart that this
is something which happened on Mars until we find organic matter in a genuine
Martian sample," he said. "We have to go to Mars and if there is doubt
we will have to bring samples back. If there is still doubt we will have to
send a person there to carry out the experiments in situ. "Our Mars
mission experiments are designed to test for organic matter and isotopic fractionation
- the best measure I know to detect organic life," he added.

Rock origins

One major objection to NASA's claim has been that the magnetite crystals could
have come from bacteria which infiltrated the rock after it landed on Earth.
Dr Friedmann says that the crystals are undoubtedly from Mars, because they
are inside other globules of rock whose origin is uncontested. The Viking missions
to Mars in the 1970s analyzed Martian rock and the Martian atmosphere. These
measurements allow scientists to work out whether a meteorite bears the chemical
signature of something formed on Mars. The meteorite in this study - known as
ALH84001 - was found in Antarctica in 1984 but no-one realized it was
a Martian meteorite until 1993.